Security readout for executives and security teams
Plain-English summary
Certain Juniper EX4300 switches can be forced to crash and restart when they receive specific valid unicast frames on vulnerable ports. The issue affects availability, not data confidentiality or integrity. Business impact is localized network disruption where affected EX4300 switches carry production access or aggregation traffic.
Executive priority
Treat as a scheduled but important network availability fix. It is not rated critical and has no cited active exploitation, but a crash-restart condition on access switches can interrupt users, sites, or operational systems.
Technical view
CVE-2021-0242 is improper DMA buffer handling in Junos OS on EX4300. On interfaces without L2PT or dot1x configured, specific valid unicast frames can leak DMA buffers until the FPC crashes and the device restarts. Fixed releases are listed across Junos 17.3 through 20.3 branches.
Likely exposure
Exposure is limited to Juniper EX4300 switches running affected Junos OS releases. The attacker must be on an adjacent network path, and the vulnerable condition depends on interface configuration lacking L2PT or dot1x.
Exploitation context
The source bundle does not report active exploitation, and CISA KEV status is false. The attack requires sending specific frames to an affected interface, so practical risk is highest on shared or less-trusted Layer 2 segments.
Researcher notes
Key triage points are model, Junos branch, interface configuration, and adjacency. Evidence supports denial of service through DMA buffer exhaustion, with no confidentiality or integrity impact stated. Do not generalize beyond EX4300 without vendor evidence.
Mitigation direction
- Upgrade affected EX4300 switches to the Junos OS fixed release for their branch.
- Prioritize switches carrying critical access, aggregation, or operational traffic.
- Review Juniper JSA11135 before changing interface configurations or maintenance windows.
- Where appropriate, assess whether L2PT or dot1x configuration changes reduce exposure.
Validation and detection
- Inventory Juniper EX4300 devices and record exact Junos OS versions.
- Compare installed releases against the affected and fixed versions in JSA11135.
- Identify exposed interfaces without L2PT or dot1x configured.
- Review device logs for FPC restart or pfex_junos crash messages.
- Monitor FPC DMA buffer usage using Juniper documented diagnostics.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
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CWE-119: Exact CWE lookup
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Open ATT&CK lookupCWE-241: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-0242 mapping review
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Open ATT&CK lookup- Severity
- Medium
- CVSS
- 6.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H2.83.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
6.5MediumVector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://kb.juniper.net/JSA11135CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Restriction of Operations within the Bounds of a Memory Buffer
Improper Restriction of Operations within the Bounds of a Memory Buffer represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Improper Handling of Unexpected Data Type
Improper Handling of Unexpected Data Type represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.